Global implementation of Automatic Dependent Surveillance (ADS) is underway and promises to replace conventional radar surveillance (e.g., Secondary Surveillance Radar, or SSR) within the next 10 to 20 years. Switching to ADS from radar techniques represents a fundamental shift in the techniques and philosophy of aircraft tracking.
In support of this transition, many aviation organizations are developing standards to ensure a proper and well-engineered transition from present radar-based systems to an ADS environment. An important part of this is the use of “Figures of Merit” or quality indicators associated with ADS solutions. ADS cannot supplant conventional radar techniques unless it can be demonstrated to be at least as accurate as prior art radar surveillance techniques. Techniques for quantifying the accuracy of ADS are thus important to the implementation of ADS to supplant radar. Techniques for measuring the accuracy of prior art radar tracking systems might not lend themselves to quantifying the accuracy of ADS.
Working groups within RTCA (Radio Technical Commission for Aeronautics, RTCA, Inc., 1828 L Street, NW, Suite 805, Washington, D.C. 20036), EUROCAE (The European Organisation for Civil Aviation Equipment, EUROCAE, 17, rue Hamelin, 75116 Paris, FRANCE), and ICAO (International Civil Aviation Organization, ICAO, 999 University Street, Montreal, Quebec H3C 5H7, Canada) have developed performance metrics for ADS, and some standards and guidelines already exist. These developed and existing standards and guidelines include: Minimum Aviation System Performance Standards for Automatic Dependent Surveillance, RTCA 242A, dated Jun. 25, 2002, Minimum Operational Performance Standards for 1090 MHz Extended Squitter Automatic Dependent Surveillance Broadcast (ADS-B) and Traffic Information Services Broadcast (TIS-B), RTCA 260A, 2003, which supersedes the earlier RTCA DO 260, and Safety, Performance, and Interoperability Requirements Document for ADS-B-NRA Application, ED-126 V1.0, December 2005, The European Organization for Civil Aviation Equipment, all of which are expressly incorporated herein by reference in their entirety.
RTCA standards define the “Figures of Merit” or quality indicators for the system. Positional data delivered by ADS-B typically depends on Global Navigation Satellite System (GNSS) receiver data. The RTCA DO260 standards require the generation and transmission of a value called “Navigational uncertainty Category” (NUC) to all ADS-B receivers, so that receivers can determine if the data is “good enough” to use.
GNSS uses satellite-positioning techniques to provide users with accurate and timely navigation information. A Global Positioning System (GPS) is a subset of a global navigation satellite system because a GPS system needs only to provide the ability to determine position information. GNSS also provides real time navigation information. In order to use GNSS for navigation, it must be possible to generate real-time navigation information fast enough for safe navigation.
When DO 260 was revised as 260A, NUC was specified in terms of NIC, NAC and SIL, defined as follows. Navigation Accuracy Category (NAC) is reported so that the surveillance application may determine whether the reported position has an acceptable level of accuracy for the intended application. Navigation Integrity Category (NIC) is reported so that the surveillance application may determine whether the reported position has an acceptable level of integrity for the intended application. Surveillance Integrity Level (SIL) is the probability that the integrity containment radius used in the NIC parameter will be exceeded.
Each of the parameters can be further subdivided for position and velocity (e.g., NUCp and NUCv). For a complete definition of each of these parameters please refer to RTCA DO 240A, incorporated herein by reference. From DO 240A, NIC, NAC, and SIL values are as presented in Tables 1-5 below.
Table 1 lists NIC values from 0 to 11 for a broad range of applications, ranging from lowest integrity (0) to highest integrity (11). Table 2 lists NAC values from lowest accuracy applications to highest application applications (i.e., LAAS or precision landing). Table 2 and Table 3 list separately the requirements for accuracy in position or velocity. Accuracy requirements are specified as 2 sigma, or 95%, meaning that 95% of the time the accuracy shall be better than that specified for each NAC value in the tables. Table 4 contains the integrity requirement, which is the probability that the target will exceed a predetermined containment radius. Table 5 contains a quality indicator that relates to aircraft barometric altimetry system performance.
A good discussion and definition of Required Navigation Performance (RNP) procedures may be found in the following documents, all of which are incorporated herein by reference
http://www.aviationmanuals.com/articles/article3.html
www.boeing.com/commercial/caft/reference/documents/RNP082400S.pdf
www.icao.int/anb/panels/acp/WG/M/M8wp/WP/WP806-ATT4.doc
http://adsb.tc.faa.gov/RFG/ADS-B-NRA%20SPR-INTEROP%20_ED-126—%20v1.0.pdf
ED-126 contains an assessment of ADS-B to Secondary Surveillance Radar (SSR) target separation requirements, and results of risk evaluations are shown in Table 6 and Table 7 from the EUROCAE document. As in the case of long range SSR to SSR separation, the relatively wide cross-range radar error distribution limits ADS-B effectiveness in supporting the above minimum required separation values of 2.9 NM or 4.2 NM.
Assurance that ADS-B based surveillance risk is at least as good as that of radar when separating ADS-B targets from radar targets requires defining ADS-B requirements so they are at least the Close Approach Probability (CAP) level obtained for radar to radar 5 NM separation. Using conservative values of NACp=6 and NIC=4 for ADS-B performance, and defining the radar cross-range error distribution, we get the results (Ccax where “x” is “s” or “w” depending on the Gaussian or wide angle error model) plotted in FIG. 1 in comparison to the reference values (Pcax) for SSR to SSR separation. Although ADS-B provides improved capability over the SSR baseline for the Gaussian SSR case, the SSR wide-angle errors at long ranges for the alternate assumption limit the incremental effectiveness of ADS-B in this case.
As defined by ED 126, the Close Approach Probability (CAP) is the probability that, when the surveillance positions of two aircraft appear to be separated by a distance S, their true separation is actually within a distance A (the size of an aircraft and typically 200 ft is used). The CAP is calculated from the assumed surveillance error distribution function (i.e. from the probability density function of the errors). In the case of radar, the position error distribution is taken for azimuth errors (cross-range errors) projected at the limit of the range of applicability of the separation minima (S). Ideally, the error distribution is determined from the analysis of real radar measurements. Note that this error distribution is for the normal operation of the radar in its particular environment with no radar equipment fault conditions (but including the tail errors caused by environmental effects). It is assumed that all the radar errors in the distribution are undetected by the radar.
If the position error distribution of a new surveillance system is known (from measurements and/or analytical predictions) then the surveillance risk can be calculated and compared to the reference radar system. Also, the surveillance risk between an aircraft position measured by existing radar and another measured by the new source of surveillance can be calculated.
In the case of (Automatic Dependent Surveillance, Broadcast) ADS-B, a quality indicator will qualify the expected accuracy of the ADS-B reported position and some integrity checking of the position is expected in conjunction. However, there may be some probability of not detecting position errors outside certain bounds and these errors will contribute to the overall undetected ADS-B position error distribution.
The above analyses show differences in SSR and ADS-B performance and essentially compare the performance metrics from both systems, which have quite different error mechanisms. The same type of analysis could be applied to multilateration and scenarios could be presented for comparing multilateration to SSR and ADS-B, as well as other technologies.
If ADS-B is to supplant SSR as a primary source of aircraft surveillance, some sort of a back-up and/or validation may be required. Many air traffic authorities recognize the need for a back-up to ADS-B and it is becoming a more common theme in aviation conferences worldwide, for example at ATCA conferences (www.ATCA.org), Maastricht ATC conferences (www.atcmaastricht.com), and Helios conferences and seminars (www.helios-is.com), all of which are incorporated herein by reference.
Surveillance alternatives generally considered for application as a back-up to ADS-B include:                Multilateration        Primary radar        Secondary Surveillance Radar (SSR)        Passive SSR        Passive Primary Radar        Other techniques including ADS-B angle of arrival from phase measurements (e.g., multi-sector antennas).        
While aviation authorities worldwide discuss the idea of a back-up as redundant forms of surveillance, or as interoperable forms of surveillance, there is no existing methodology to combine the different sources of surveillance into an overall surveillance service with a common set of performance metrics. For example, in the United States, the Federal Aviation Administration (FAA) has revealed ambitious plans to commence with a national ADS-B program (www.faa.gov). The FAA is interested in back-up surveillance, and will currently consider performance-based approaches, which are not technology specific. However, in the Prior Art, there exists no methodology to combine data from different surveillance sources, based on the categorized quality of data
SSR and ADS-B have different methodologies for tracking aircraft. However, as noted above, performance metrics for ADS-B have been established such that the accuracy and integrity of an ADS-B system can be readily determined. Back-up methodologies may require similar accuracy and integrity standards. However, again, since the methodologies differ in their underlying technology, creating equivalent metrics for monitoring such back-up systems may be difficult. Thus, it remains a requirement in the art to provide a system and method for monitoring and measuring metrics of a back-up methodology such as multilateration and presenting such metrics in the same or similar terms as ADS-B metrics.